1. Field of the Invention
The present invention relates to a transponder for an RFID system, an RFID system for wireless data exchange comprising a transponder and a base station, and a method for requesting write and/or read access to a transponder, in particular a passive and/or backscatter-based transponder.
2. Description of the Background Art
A radio frequency identification (RFID) system typically has a base station or a reader (or reader unit) and a plurality of transponders or remote sensors, which are located in the response area of the base station at the same time. The transponders and their transmitting and receiving devices customarily do not have an active transmitter for data transmission to the base station. Such non-active systems are called passive systems if they do not have their own energy supply, and semi-passive systems if they have their own energy supply. Passive transponders take the energy they require for their supply from the electromagnetic field emitted by the base station.
In general, backscatter coupling is used to transmit data from a transponder to the base station using UHF or microwaves in the far field of the base station. To this end, the base station emits electromagnetic carrier waves, which the transmitting and receiving device in the transponder modulates and reflects appropriately for the data to be transmitted to the base station using a modulation method. The typical modulation methods for this purpose are amplitude modulation, phase modulation and/or amplitude shift keying (ASK) subcarrier modulation, in which the frequency or the phase position of the subcarrier is changed.
An access control method for transponders is described in the proposed standard ISO/IEC_CD 18000-6C dated Jan. 7, 2005, (see in particular ISO/IEC_CD 18000-6C section 6.3.2, pages 27-60). In this method, the transponder is first selected from among a plurality of transponders in a selection or arbitration process. The selection method described is a stochastic method in the form of a slotted ALOHA method. Such selection methods are described in detail in, for example, the “RFID Handbuch,” a textbook by Klaus Finkenzeller, HANSER Verlag, third edition, 2002 (see in particular section 7.2, Vielfachzugriffsverfahren—Antikollision [multiple access methods—anticollision], pages 203 to 216).
An access control method is likewise described in, for example, DE 10 2006 002516 A1, which corresponds to U.S. Provisional application No. 60/838,889, and which is incorporated herein by reference.
To carry out an access operation, a reader typically transmits what is called a query command. The transponder responds to this query by transmitting a random number. The transmission of an acknowledge message isolates the transponder. The isolated transponder transmits protocol control bits (PC) and an identifier in the form of an electronic product code (EPC) to the reader. The protocol control bits contain information regarding a physical layer of the transmission path. Among other things, the identifier or the electronic product code EPC reflects a product identified by the transponder. The assignment of the EPC to the identified product is standardized, so that the product can be deduced from knowledge of the EPC.
After the transponder has transmitted the PC and EPC, read and/or write access to memory areas of the transponder by the base station is typically possible, unless certain areas are protected or locked by a password against read and/or write access. Moreover, read and/or write access to another memory area of the transponder can be permanently locked against direct access by the base station through an air interface and/or a PIN. Stored in this so-called private area are passwords, for example, wherein the memory area is accessible for a password comparison by an internal logic unit of the transponder, for example.
For write access to a password-protected area, one or more passwords must be transmitted by the querying base station to the transponder. It is customary for a symmetric method to be used for this process, i.e., the same passwords are stored in the base station and the transponder. A simple protection system is created in this way. However, a disadvantage of symmetrically distributed passwords or keys is the considerable effort required for password or key management. For access to a transponder, the associated password must be distributed to all participating base stations or readers. Especially in the case of data management systems which must be flexibly adaptable to changing infrastructures and/or applications, known as open data management systems, a considerable amount of effort would be required here for key distribution.
In addition, so-called asymmetric methods are known, in which data are encrypted by what is known as a public key, which is accessible to an unlimited number of participants, whereas only one participant or a limited number of participants knows a secret for decryption. Asymmetric methods are customary for authentification and/or authentication in the field of money transfers, for example. However, asymmetric methods are generally computationally intensive, and require a battery-operated transponder or another battery-operated unit.